The present invention relates to a detection system having an optical encoder to optically detect a displacement such as the rotational speed and rotational angular position of a movable device.
Rotation detecting system of the type of having an optical encoder to optically detect rotation, or rotational displacement, of a rotating device such as a motor and a rotating table is known. The optical encoder generally includes a disc-like member rotatable in accordance with rotation of the rotating device and having a plurality of slits formed circularly at an equal angular interval on the vicinity of its circumference. The rotatable disc-like member is interposed between a light-emitting device and a light-receiving device of an optical pickup apparatus so that light emitted from the light-emitting device is intermittently incident through the slits of the rotatable disc-like member onto the light-receiving device in response to rotation of the rotating device. The light-receiving device is adapted to successively generate an electrical signal with a level corresponding to the quantity of light incident thereon and the generated electrical signals are used to detect the rotational speed or rotational angle of the optical encoder and then to effect rotation control of the rotating device. One of the major requirements arising in rotation detecting systems is the improvement of detection accuracy. One possible solution is to increase the number of slits to be formed on the rotatable disc-like member to reduce the pitch between the slits. However, in such a conventional rotation detecting system, excessive increase in the number of the slits can reduce the quantity of light from the light-emitting device to the light-receiving device, resulting in making it difficult to detect the intermittent variation of light and thus providing design limitations imposed on the increase in the number of the slits to be formed in the optical encoder.